Cicadas feed exclusively upon tree sap. Most of their feeding behavior occurs during their extended larval stage, during which they live underground and suck sap from the roots of trees. It is common folklore that adult cicadas do not eat, but this is not true.
Adult cicadas also eat tree sap, although they live above ground during this life stage and thus rely upon fluid from the trunk or branches rather than sucking sap from the roots. However, it is true that the adult forms of certain insects do not eat. For example, the luna moth does not even possess a mouth. Cicadas are only distantly related to these insects.